1. Field of the Invention
The invention relates to a pivot device between parts of an orthopedic aid, for example a prosthetic knee for leg amputees, comprising a multi-element kinematic joint chain with at least four joint members, in which the members connected with one another have a common rotational axis, and these rotational axes extend essentially parallel to one another.
2. Description of the Prior Art
The use of multi-element joint systems for replacement or reinforcement of joint function in orthopedic aids has already been known for a long time and is frequently employed because of the advantages that such a mechanism enjoys over more conventional mechanisms with a single fixed pivot (center of rotation). These advantages include among others an improved following or improved imitation, in comparison to a single-axis joint mechanism, of a natural movement of the limbs and an increased or better-controllable joint stability under load and are achieved by a suitable choice of the dimensions of the members of the joint system and their positions with respect to one another. In particular, when a multi-element joint system is used to replace or reinforce the knee function, the latter aspect is of great significance. At the beginning of a step, when the heel touches the ground and the joint mechanism in the extended position is loaded by the weight of the user, it is important that the mechanism not bend immediately because the user would then have no support for the body and would fall. This can be prevented in a single-axis joint mechanism only by using a complex and less reliable brake mechanism.
In a multi-element joint system this property on the other hand can be achieved by choosing the joint geometry in such fashion that in the extended state of the joint mechanism, the virtual rotational point around which pivots the lower leg or the part of the leg prosthesis that replaces the lower leg, lies behind the line of application that connects the two loading points, namely the heel and the hip joint.
By a suitable choice of the geometry of the multi-rod mechanism, it is also possible to ensure that the virtual point of rotation of a stepping movement is located such that the prosthesis or orthesis can be brought into a forwardly pivoting movement more simply in comparison to a single-axis mechanism. The great stability to be achieved at the beginning of a stepping movement, the easily conferred backward swinging movement, and the possibility of finding a good compromise between these two properties in a simple fashion make the multi-rod mechanism suitable simply for replacement or for reinforcement of a joint function.
An important requirement for a kinematic multi-rod mechanism in such applications is that it have an initial deflecting bend under load, as for example the human knee joint also does under load. The cushioning effect of this initial deflecting bending of the joint under loading of the leg prevents a jerky gait which can be uncomfortable for the user and even painful with time. The initial bending of the joint also prevents a limitation of the vertical movement of the center of gravity of the body so that the energy required for walking remains limited. This knee bending is referred to as "stance flexion."
The existing pivoting devices between parts of an orthopedic aid, for example a prosthesis for leg amputees, suffer from at least one of the following disadvantages:
They do not exhibit the properties described at all.
They possess the described properties only to a slight extent. This is the case when the stance flexion that occurs in practice is no greater than 3.degree. to 4.degree. which is too limited for the described advantages of stance flexion to be completely realized.
They produce the described properties by using a multi-element joint system with two degrees of freedom, both of which can be understood as the above mentioned pivoting movement of the mechanism, with the movement taking place according to a single degree of freedom against the spring force of an elastic element and with the two degrees of freedom influencing one another mutually, but nevertheless being independent of one another. These mechanisms have the disadvantage that the exchange of stored spring energy between these degrees of freedom is always possible, and this is perceived by the user as an extremely unpleasant effect so that additional means, for example dissipative elements, are required in order to convert this spring energy into heat by dissipation.
They provide the described properties by using a multi-element joint system with only a single degree of freedom and one turning point in the pivoting movement, which is achieved by virtue of the fact that in the extension of one of the members of the multi-element joint system a second member or a push-on connection is provided that makes such mechanisms relatively long.